1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the same.
2. Description of the Related Art
Drive systems of a liquid crystal display element used for a liquid crystal display device (LCD) include an analog drive system and a digital drive system. In the analog drive system, a voltage value applied to a pixel is a continuous analog value. In the digital drive system, the magnitude of a voltage applied to a pixel is set to be binary, and duration of an applied voltage is changed depending on the brightness (grayscale) of an image. Accordingly, an effective voltage value applied to a liquid crystal pixel is controlled. In the digital drive system, only information of “0” or “1” (subframe data) is applied to a pixel. Therefore, the digital drive system has a feature to be resistant to the influence of an external factor such as noise.
Moreover, if a liquid crystal such as a TN (Twisted Nematic) mode or a VA (Vertical Alignment) mode is used, polarity reversal drive (hereinafter also referred to as the DC-balanced drive) is used. This is for preventing a liquid crystal burn-in phenomenon where if a voltage is applied to a liquid crystal in a specific direction for a long time, the liquid crystal is ionized so that brightness adjustment by a voltage, which is intrinsic to the liquid crystal, becomes impossible to be made. In the DC-balanced drive, the voltage applied to the liquid crystal is alternately reversed in positive and negative directions, and a DC component of the voltage applied to the liquid crystal can be cancelled. Consequently, long-duration application of the voltage to the liquid crystal in a specific direction can be avoided and accordingly the DC-balanced drive is important from the viewpoint of reliability. Therefore, in a liquid crystal display device of the digital drive system, it is also necessary on a subframe basis to avoid applying a voltage to a liquid crystal in a specific direction for a subframe period or more, in order to prevent liquid crystal burn-in between a pixel electrode and a common electrode.
In the digital drive system, for example, one frame is divided into a plurality of subframes having a shorter display period than one frame period, and pixels are driven in a combination of subframes selected in accordance with a grayscale level to be displayed from the plurality of subframes (for example, see Japanese Laid-open Patent Publication No. 2006-171651). In the digital drive system, as described above, only information of “0” or “1” is applied to a pixel. In other words, that is only information whether to display black or white. Hence, it is necessary to generate grayscale levels by temporal integration. In the grayscale generation method described in Japanese Laid-open Patent Publication No. 2006-171651, one frame period is divided into a plurality of subframes, and periods during which the subframes can perform display are set to periods having a ratio of “1”, “2”, “4”, “8”, “16”, “32”, “32”, “32”, “32”, “32”, “32”, and “32.” For example, if a grayscale level of “100/255” is desired to be displayed, information, [001001110000], is written to a pixel in turn, and accordingly a subframe having a display period “4” and three subframes having a display period “32” are selected to represent the grayscale level of 100/255.
Moreover, known as the above liquid crystal display device of the digital drive system is one where each pixel includes two sample holding units connected in series, one voltage selection unit, and a liquid crystal display element. In the pixel, one-bit data held in the sample holding unit in a previous stage is transferred to a sample holding unit in a following stage at timing when a common signal is supplied via a common transfer signal line, and held therein. One of two types of voltages, V0 and V1, is selected at the voltage selection unit in accordance with the value of the held data, and applied to a pixel electrode of the liquid crystal display element. The liquid crystal display device including the pixel can collectively transfer a screenful of data to the sample holding unit in the following stage, and can also be applied to a three-dimensional image display device.
Moreover, a liquid crystal display device excluding the voltage selection unit from the configuration of the pixel and including a pixel where two latches are arranged in series is conventionally known as the liquid crystal display device of the digital drive system (for example, see Japanese Laid-open Patent Publication No. 2001-523847).
In recent years, with higher resolution and downsizing of liquid crystal display devices, pixels are further required to be downsized. Moreover, a demand for a three-dimensional image display device is becoming stronger, and a demand for a display device capable of rewriting a screen in one batch is also becoming stronger.
However, the known liquid crystal display device described in Japanese Laid-open Patent Publication No. 2001-523847 has a problem in the grayscale generation method. For example, data is transferred as schematically illustrated in (A) of FIG. 15 to perform display with each of the above-mentioned subframes, and a liquid crystal display element is driven for periods illustrated in (B) of FIG. 15. Accordingly, a data transfer suspension period that is not used for the transfer of data becomes long. Hence, the known liquid crystal display device described in Japanese Laid-open Patent Publication No. 2001-523847 has problems of very low efficiency and resulting cost increase.
Moreover, the known liquid crystal display device is capable of shortening the data transfer suspension period, but has a problem that a drive suspension period is required instead so that the display becomes dark. Moreover, there is also a problem that the downsizing of a pixel is difficult compared with the known liquid crystal display device described in Japanese Laid-open Patent Publication No. 2001-523847 since the voltage selection unit is included in a pixel.